1. Field of the Invention
This invention relates generally to a process and apparatus for producing reform gases. More particularly, this invention relates to a process and apparatus for producing reform gases to boost the reformed gas capacity of existing reformers.
2. Background
Processes for making reformed gases are widely used throughout the world and have particular application in connection with Direct Reduced Iron (DRI) plants. The DRI plants use reformed gases in large quantities to reduce iron ore (FeO) into iron (Fe) inside a shaft furnace. The iron from the shaft furnace is then processed into various steel grades for fabricating final products such as wires, rods, beams, and the like.
The reformed gases used in such shaft furnaces are mainly a hydrogen (H.sub.2) and carbon monoxide (CO) mixture in the general proportion of 1.5:1, respectively. These reforming gases (H.sub.2 and CO) provide the following metallization reactions in the shaft furnace: EQU FeO+H.sub.2 .fwdarw.Fe+H.sub.2 O EQU FeO+CO.fwdarw.Fe+CO.sub.2
The stoichiometric calculations of the above reactions indicate that 400 cubic meters of CO or H.sub.2 react with the iron ore (FeO) to form 400 cubic meters of CO.sub.2 or H.sub.2 O for each metric ton of iron reduced from FeO. Chemical calculations require that the ratio of the reductants (CO+H.sub.2) to oxidants (CO.sub.2 +H.sub.2 O) be greater than about 2:1 before any reduction occurs. Therefore, the reformed gas entering the shaft furnace must have a sufficient amount of reductants to allow conversion of 400 cubic meters of reductants into oxidants per ton of iron, and still have a ratio of 2:1 of reductants-to oxidants after all the FeO is reduced to Fe.
The metallization process is carried out in the shaft furnace in which iron oxide is fed from the top in a feed hopper and distributed in the furnace by several distribution legs. The shaft furnace has three zones in which the process is carried out; a reduction zone, a transition zone and a cooling zone. A bustle (larger diameter section), is provided on the furnace which has ports opening into the bottom of the reduction zone through which the bustle gases are injected into the furnace to pass upward through a bed of iron ore in the reduction zone. The bustle gas is mainly the reformed gases (H.sub.2 and CO) plus CO.sub.2, H.sub.2 O and enrichment natural gas. A typical bustle gas composition is as follows: EQU CO.sub.2 =02.5% EQU CO=38.0% EQU H.sub.2 =56.0% EQU CH.sub.4 =02.0% EQU N.sub.2 =01.5%
The nitrogen is present due to air entering the process at various points, and also due to the fact that the natural gas used in the process may contain up to about 2% nitrogen. The typical bustle gas temperature is around 1650.degree. F. This temperature is achieved after the addition of enrichment natural gas to the reformed gas from the reformer.
The enrichment natural gas is added to provide a source of carbon in the reduction zone. This permits addition of carbon to iron in the reduction zone by the carburization reaction as follows: EQU 3Fe+CH.sub.4 .fwdarw.Fe.sub.3 C+2H.sub.2
This reaction is endothermic and reduces the iron oxide bed temperature. The amount of iron oxide feed, bustle gas temperature, H.sub.2 /CO ratio and the amount of reformed gas CO.sub.2 are largely controlled by the plant operation control loop. The most effective carbon-controlling techniques currently practiced involve adding controlled amounts of natural gas to the bustle gas. Many plants operate at about 2.5% to about 3.5% CH.sub.4 in the reducing zone from the bustle gas and about 20.0% to about 50.0% CH.sub.4 in the cooling zone. However, the addition of the CH.sub.4 as the enrichment gas to the reformed gases entering the bustle tends to reduce the bustle gas temperature making control of the bustle gas temperature more difficult.
The traditional reformed gas-making process for DRI plants is carried out in a reformer where a hydrocarbon feed gas such as natural gas, methane, propane and the like is reacted with H.sub.2 O and CO.sub.2 (at approximately about 1900.degree. F. to about 2000.degree. F.) in the presence of a catalyst to produce the reductants CO and H.sub.2.
The central piece of equipment is a furnace consisting of a refractory-line shell containing catalyst-filled reforming tubes. Fuel is burned in the shell at slightly over atmospheric pressure, while a mixture of natural gas, H.sub.2 O and CO.sub.2 are passed through tubes containing catalyst pellets made from such material as nickel or alumina nickel. The reforming reactions are characterized by being endothermic (requiring heat input) and by requiring a catalyst to accelerate the reforming reactions. Accordingly, multiple burners are fired in the shell to provide the necessary heat input.
The feed gas (natural gas, methane or propane) is fed to the reforming tubes from an external source while CO.sub.2 is provided to the reforming tubes in the form of exiting furnace gas from the shaft furnace. The necessary water (H.sub.2 O) is added prior to the gaseous mixture entering the reforming tubes. The feed gas and the H.sub.2 O and CO.sub.2 are mixed and heated in the catalyst-filled reforming tubes to cause the following two reforming reactions to occur in the reforming tubes: EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2,
and EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2.
A typical reformed gas leaving the reformer gas tubes may have a temperature of about 1700.degree. F. and the following composition (on a dry basis): H.sub.2 =58.0%; CO=38.0%; CO.sub.2 =2.5%: CH.sub.4 =0.5%; and N.sub.2 =1.0%. The quality of the reformed gas is defined by the reductants (H.sub.2 +CO) to oxidants (CO.sub.2 +H.sub.2 O) ratio, the higher being the better. A typical value for the reductants-to-oxidants ratio is about 12 with a H.sub.2 /CO ratio of 1.5 to 1.
Various factors affect the quality and the reformed gas flow rate. Such factors include the reformer output, reformer tube temperature and reformer burner size. If the reformer output (reformed gas flow rate) is increased beyond the reformer design capacity, the heat load supplied by the reformer burners also increases. As the heat load increases, the catalyst in the center of the reformer tubes gets colder due to the increased heat removal rate through the tubes. The colder catalyst tends to increase the potential of undesirable carbon deposits in the tube, thus reducing the overall reforming performance. Both quality (composition) and flow rate of the reformed gas would be seriously affected due to operating above the designed capacity.
An increase in reformer temperature will also increase the reformer tube temperature. This may set up thermal stresses and possible distortion and damage to the reformer tube material (typically silicon carbide). A damaged reformer tube could cause a complete plant shutdown and loss of production.
The reform burners are sized for a certain firing capacity and flame characteristics. The firing beyond the reformer design capacity would provide an unacceptable temperature profile along the reformer tube length and a potential overheating of the reformer furnace shell refractory. A 2200.degree. F. limit is typical for the reformer. An unacceptable temperature profile along the reformer tube length could affect the catalyst activity and reforming action within the tube, and result in a deterioration of the reformed gas quality. This could result in lower metallization rates by the shaft furnace and/or the production of poor quality reduced iron.
Because of the above, most direct reduction plants are unable to increase their capacity of the reformed gases at will from the reformer beyond the capacity of the reformer. On the other hand, the shaft furnace is generally capable of increased throughput of 20% to 30% of the reduced iron if additional reformed gases could be supplied for the metallization reactions inside the furnace. If a plant desires increased output from its shaft furnace, the options left for the plant are to either install a new bank of reformer tubes and a reformer furnace or purchase the reduced iron from another supplier. Neither of these options are cost-effective. The cost of a new bank of reformer tubes and a reformer furnace requires a capital expense of millions of dollars and such new reformer tubes may not be necessary at all times due to market demand and overall flexibility in the production cycle. The purchase of the reduced iron from another supplier is subject to changing market pricing and availability and also does not present a satisfactory solution to increasing output of the reduced iron.
In view of the above, it is desirable that a relatively low-cost option be provided that would increase the availability of reform gases upon demand to permit an increase in the production of reduced iron when market conditions demand, and which can be cut back to the original capacity of the existing reformer when market conditions dictate a reduced output.